One type of side-scattering light guide is described in U.S. Pat. No. 6,169,836 (Sugiyama). This patent describes a light guide formed with scattering particles dispersed in the core. The scattering particles have a significantly different refractive index from the core material and therefore cause relatively large scattering angles. The scattered light from this invention has an aesthetically unpleasing appearance.
U.S. Pat. No. 5,542,017 (Koike) describes a side-scattering light guide that has a core containing an irregular structure of regions of small refractive index differences produced from a partly phase separated co-polymer during the polymerization process. Although the scattered light has a more aesthetically pleasing appearance, the process of producing such a light guide is particularly difficult to control.
The Applicant's co-pending International patent application PCT/AU02/00631 describes flexible polymeric side scattering light guides using transparent diffuser particles that are closely refractive index matched with the core bulk material. A refractive index mismatch of a few percent is typical.
Methods of manufacturing side-scattering light guides comprising light scattering additives in the form of diffuser particles are also disclosed in the co-pending International patent application, and its contents are incorporated by reference.
These light guides transport light efficiently in the axial direction, that is from end to end. The fraction of light that is side scattered out in a given length of fibre is determined by the number density of the diffuser particles in the core, but even with densely seeded cores the light guides only glow faintly when viewed from the side, and they give almost no light in the backward direction.
An additional problem is that when viewed from the side, the edges of the guide appear to be much brighter than the centre. The centre also appears to have a slightly “milky” appearance. Many observers perceive the brightness as less than an objective measure would indicate, and the overall impression is aesthetically unsatisfactory.
A further problem is that side-scattered light tends to be emitted in a radially symmetric fashion and a substantial fraction of it may be in unwanted directions.
An additional problem is that for some types of diffuser particles the fraction of light scattered out per meter of fibre varies with the light's wavelength. If white light is used in such fibres the side-emitted light is coloured and the colour changes along the length of the fibre. Colour variation of side-emitted light along a light guide, particularly one using white light, is unacceptable for many applications.